I've never used the 150 but the Ace-250 is about as good as it gets in that price range. I've found lots of silver coins with one and it's very hot on silver, copper, and modern clad coins. It's not as good on nickels or gold rings but a lot of folks still find them just fine. They tend to be very durable machines so I wouldn't have a problem buying used if you can find a good deal.
The big advantages of the 250 is you can adjust the discrimination how you want and notch certain ranges in or out to create custom settings instead of just the pre-programmed settings. Also the 250 had a pinpoint mode that helps find the exact location of the object and saves digging time.
The Fisher F2 is about the same price range as the Ace-250 with similar features but it doesn't go as deep on silver and modern clad coins from what I've seen using them side by side on the same targets in the soils around here. The F2 does seem to do a little better on nickels and gold rings though and the discrimination of shallow coins in trash is pretty good. It gives up about 1/2" to 3/4" to the Ace 250 on a buried silver dime though.
The Tesoro Compadre and Silver uMax are good lower-price detectors too.
